TROUBLE GOING UP HILL

1980 Porsche 911

Engine Performance problem
1980 Porsche 911 6 cyl Two Wheel Drive

My husband has a 1980 Porsche 911 Targa SC - very sluggish going uphill/inclines. He recently changed the fuel filter, spark plugs, cap, and rotor. Still not performing as well as it should. What do you think it could be?